The present invention relates to an apparatus for the continuous measurement of the heating power of a fuel gas, allowing the slow variations of this power to be continuously monitored.
Numerous apparatuses are already known for continuously measuring the heating power or the combustion heat of fuel gases, such as gases from natural deposits. Such installations are more especially used in cooperation with a system for measuring the flowrate in gas lines for determining the heat energy supplied to customers.
Most prior art measuring installations use thermometric detectors formed by transducers delivering an electric signal. The use of these transducers results in serious defects. The need for insulation introduces thermal resistances, the ageing of which adversely affects performances and reliability. The small area of the active surfaces of the individual transducers makes it difficult to attain a distribution over the path of the heat flux supplying a satisfactory averaging and integration of the flux densities over the whole transfer surface. The latter defect is particularly serious when the flux to be measured passes over a large-size exchange surface and presents a heterogeneous and random density distribution.
It is an object of the invention to provide an improved continuous measurement apparatus; it is a more specific object to provide an apparatus which allows satisfactory integration of the flux over the whole transfer surface area and achieves satisfactory accuracy with relatively simple means.
To this end, there is provided an apparatus for the continuous measurement of the heating power of a fuel gas, comprising a calorimeter in which is disposed an open cell whose bottom part surrounds a burner and means for supplying the burner and the cell respectively with fuel gas and combustion air at a given pressure and feedrate, characterized in that the calorimeter comprises an external wall maintained at constant temperature and defining, with the wall of the cell, an annular space divided, by means of a dividing wall parallel to the walls, into an internal enclosure and an external enclosure of small thickness with respect to their other embodiments, said enclosures being occupied by the same gas and associated together so as to form a differential gas thermometer.
It can be seen that with this arrangement any thermal contact resistance is avoided since the gas in contact with the wall of each enclosure has the same temperature as the wall of the cell, on the one hand, and the external wall, on the other. Since the thickness of the enclosures, so of the gas ducts which they contain, is small, the temperature gradient in these enclosures will be substantially normal, that is to say radial if the enclosures, and so the walls, are of revolution about an axis. The calorific inertias brought into play are small, so that the installation is capable of following the variations of heat flux produced by a flame of natural gas whose flowrate is constant whereas its composition is slowly variable.
The installation may be readily provided with calibration means. These latter may comprise a heating resistance whose effect is substituted for that of the burner. It is however preferable to use means for alternately supplying the burner with a fuel gas whose characteristics are to be measured and with a reference gas, for example pure methane when the installation is intended for measuring the heating power of natural gas.
To prevent the airflow sucked in from being appreciably affected by heating power variations of the fuel gas, it is sufficient in practice to construct the cell in the form of a duct topped by a discharge chimney having an electric heating resistance releasing a power much greater than the flux to be measured; said duct being provided with baffles for braking the air stream drawn in by convection. Thus, the heat flux variations have no real influence and there is created in the discharge chimney a natural convection current practically independent of the flux to be measured.
The gas thermometer formed from internal and external enclosures may be provided in the form of a pneumatic measuring bridge supplied by a gas source at a variable pressure following an alternate periodic law. In particular, a pneumatic measuring bridge may be used of the kind described and claimed in French Patent Publication No. 2,514,128.
The invention will be better understood from the following description of a particular embodiment, given by way of examples only.